1. Field of the Invention
The invention relates generally to motors and more particularly to an electronically commutated direct current motor which generates adequate starting torque.
2. Description of the Prior Art
These types of motors, which are described for example in the German patent application No. P 33 14 239.4 (Glasauer et al, now U.S. Pat. No. 4,558,245) are single-stranded coil type two pulse current supplied, commutatorless and permanently energized direct current (dc) motors having an external rotor. Upon supplying the stator winding with current, the dc motors are capable of being restarted automatically from the standstill if only a small braking torque exists at the motor shaft. The supply of current to the stator winding is effected in these types of dc motors in dependence upon the rotational position of the rotor. The rotor position is ascertained by a sensor, such as a Hall generator, with a signal obtained from the sensor utilized to control an electronic circuit which applies a source of direct current voltage to the stator winding for forming the required starting torque. In these types of dc motors aided by the electronically controlled stator winding, a stator alternating current field is provided which in cooperation with the exciting field of the permanent magnet, forms a pulsating moment. In cases where both the polar axis of the permanent magnet and the polar axis of the alternating current field are not displaced with relation to one another, the starting torque equals zero. Under these conditions, the rotor is prevented from starting on its own. The forming of a starting torque requires an angle differing from zero to exist between the polar axis of the permanent magnet and the polar axis of the stator alternating current field. This means that in the case of a currentless stator winding, the polar axis of the permanent magnet must be displaced by a certain angle of rotation with respect to the polar axis of the stator alternating current field.
In the dc motor proposed in the German patent application No. P 33 14 239.4 (now U.S. Pat. No. 4,558,245), the necessary displacement (twist) of the rotor with respect to the stator when the motor is in the rest position is realized in that each pole stack is provided on the same side with an extension extending in the circumferential direction. Therefore, to the rotor, the stator offers a non-constant magnetic resistance along the circumference of the stator. In the rest position of the motor, the rotor assumes such a position with respect to the stator that an angle differing from zero is formed between the longitudinal stator axis and the polar axis of the permanent magnet. For assuring the reliable starting of the dc motor, it is required for the rotor to have only one rest position. In the case of the dc motor as proposed in the aforementioned German patent application No. P 33 14 239.4 (now U.S. Pat. No. 4,558,245), it is possible that the rotor can assume a second rest position from which the dc motor is not capable of automatically starting.